Electric cable



Jan. 2s, l1936. T l R. SCOTT ETAT 2,029,038

ELECTRIC CABLE Filed April 6, 1952 FlG. 3A FIG. 3B

INVENTORS THOMAS R. SCOTT Patented Jan. 2,8, 1936` UNITED STATES PATENTOFFICE ELECTRIC CABLE Application April 6, 1932, Serial N0. 603,471 InGreat Britain vJulie 18, 1931 claims. (c1. 173-266) This inventionrelates to cables for electric power transmission and more specificallyto cables of this type which employ an oil impregnated dielectric.

In these cables it has been suggested heretofore to prevent thedistortion of the cable parts due to thermal expansion by means of tubesor envelopes 4in or adjacent the dielectric adapted to accommodate theexcess of oil during expansion and to expand on cooling to ll out thespaces left by thecontracting iiuid.

In these compensating devices the expanding or contracting of the tubesor envelopes was invariably eiiected by the difference in pressurebetween the inside and the outside of the envelopes.

According to this invention there is provided a temperature controlledfluid pressure equalizer for an electric power cable.

According to a feature of the invention there is provided, in a fluidimpregnated dielectric, a device having a negative temperatureco-eicient of cubical expansion. Thus a iluid impregnated dielectric mayhouse a device which compensatingly contracts as the pressure in thecable increases due to the heating of the parts. A device of thischaracter may be constituted by a plurality of metallic walls togetherforming anv enclosed envelope. The structure oi one or more of the Wallsmay be bi-metallic, comprising metals or alloys having diiierentco-emcients of thermal expansion so that a change of shape of the devicefollows any change in temperature. By suitably shaping the device it maybe arranged that a change in shape produces a. change in the volumeoccupied by the device, such change in volume being much greater thanthat due merely to the normal expansion or contraction of the materialused and in general in the opposite direction.

The invention will be more fully described with reference to theaccompanying drawing ci which Fig. 1 illustrates a cable havingcompensating envelopes according to the present invention. Figure 2illustrates in detail a particular form of bimetallic envelope. Figures3A and 3B show crosssection views of a bimetallic tape in a normal andexpanded condition respectively. Figures 4A and 4B show cross-sectionviews oi a modified form of compensating device in a normal and expandedcondition respectively, mounted inside a hollow conductor.

In Figure 1 a conductor (I) has applied about it a body of insulation(2) surrounding which is a plurality of bi-metallic envelopes (3)sprally laid. These lil-metallic envelopes may take the form of hollowtapes as shown in Fig. 3A. The usual lead sheath (4) is provided. Thecable is impregnated with insulating oil or compound in known manner.Although the layer of bi-metallic tubing is shown in this embodiment asoc- 5 cupying a position outside the insulation it will be appreciatedthat it may be arranged in any other convenient position in relation tothe insulation (e. g. between the conductor and the insulation).Bi-metallic tubing may also be used as an intersheath in addition to itsnormal` function of iiuid pressure equalizing.

In Figure 2 a oi-metallic hollow tape is shown formed in links, the fulland dotted lines indicating the normal or cold condition, and the dot l5dash lines indicate the Way in which the walls of the envelope distorton temperature increase. The walls of each envelope are of a loi-metalwith the metal having the greater co-efcient of eX- pansion on theinside.

On heating of the cable with consequent expension of the iiuiddielectric there would be distortion of the cable parts unless spacewere provided for accommodating the expanded fluid. The bi-metallicstrips distort to provide accommodation-for the surplus iluid so that noundue distortion in the other parts of the cable can take place.

Where the bi-metallic envelopes are in continuous form as illustrated inFigure 1 they may 30 be constituted as shown in Figire 3A. In thisiigure the two dissimilar metals forming the walls of the envelope areindicated at a and b and Fig. 3B shows the position taken up by thewalls when subjected to an increase in temperature.

In Figure 4A the bi-metallic compensating device (3) is mounted at theinterior of a hollow conductor (i) which is oil filled. In this case oneor more of the walls d, e, f may be bi-metallic and assuming that d andf are so made, Fig. 4B 40 shows the shape taken on heating.

It will be appreciated that the compensating envelopes may take one of anumber of possible shapes. l e y The metals 'used in the construction ofthe bi- 45 metallic strip may be any having appropriate co-ecients ofthermal expansion. For example, they may be of aluminium and brass.Furthermore, it will be understood that although the lai-metallic hasbeen used, any suitable com- 50 bination of metals giving the sameeffect may be used, for example, an arrangement of three metals havingdierent co-eiicients of expansion might be used.

.it will be appreciated that instead of the bin 2 aoaaose 15 of atriangular shaped tube and so located relative to the Aoil as toautomatically contract in volume in response to a rise in `temperaturethereby to permit an increase in volume of oil without a substantialincrease in pressure.

20 3. An electric power cable having a sheath, a conductive core, uidinsulating means and a uid pressure equalizer in contact with said uidinsulating means within said sheath, said equalizer being so locatedrelative 'to the uid insulat- 25 ing means and having a wall thereof' soconstructed as to automatically contract upon a rise in temperature andthus reduce the volume of the equalizer thereby to permitan increase involume of said ud insulating means without a substantial increase inpressure. 5 4. An electric power cabley having a sheath, a conductivecore, a uid dielectric 4and a uid pressure equalizer in contact withsaid dielectric within said sheath, said equalizer comprising aplurality of hollow strips and so located relative to 10 said dielectricand constructed of such material that the hollow strips automaticallycontract in volume in response to a rise in temperature thereby topermit an increase in volume of the dielectric without a substantialincrease in pressure. l5 5. An electric power cable havingaI sheath, aconductive core and a fluid dielectric within said sheath, and means,also within said sheath and in contact with said dielectric which whensubjected to an increase in temperature automatically con- 20 tracts involume permitting an increase in volume of the dielectric without asubstantial increase in pressure.

THOMAS R. SCO'II'. THOMAS E. D. MENZIES. 25

